1. Field of the Invention
The invention is related to the production method of group III-nitride wafers using the ammonothermal method combined with cutting and processing of an ingot to improve the crystal quality from an initial group III-nitride seed.
2. Description of the Existing Technology
(Note: This patent application refers to several publications and patents as indicated with numbers within brackets, e.g., [x]. A list of these publications and patents can be found in the section entitled “References.”)
Gallium nitride (GaN) and its related group III alloys are the key material for various opto-electronic and electronic devices such as light emitting diodes (LEDs), laser diodes (LDs), microwave power transistors, and solar-blind photo detectors. Currently LEDs are widely used in cell phones, indicators, displays, and LDs are used in data storage disc drives. However, the majority of these devices are grown epitaxially on heterogeneous substrates, such as sapphire and silicon carbide. The heteroepitaxial growth of group III-nitride causes highly defected or even cracked films, which hinders the realization of high-end optical and electronic devices, such as high-brightness LEDs for general lighting or high-power microwave transistors.
Most of the problems inherent in heteroepitaxial growth could be avoided by instead using homoepitaxial growth with single crystalline group III-nitride wafers sliced from bulk group III-nitride crystal ingots for homoepitaxy. For the majority of devices, single crystalline GaN wafers are favored because it is relatively easy to control the conductivity of the wafer and GaN wafers will provide the smallest lattice/thermal mismatch with device layers. Currently, however, the GaN wafers needed for homogeneous growth are extremely expensive compared to heteroeptiaxial substrates. This is because it has been difficult to grow group III-nitride crystal ingots due to their high melting point and high nitrogen vapor pressure at high temperature. Growth methods using molten Ga, such as high-pressure high-temperature synthesis [1,2] and sodium flux [3,4], have been proposed to grow GaN crystals. Nevertheless the crystal shape grown using molten Ga is a thin platelet because molten Ga has low solubility of nitrogen and a low diffusion coefficient of nitrogen.
An ammonothermal method, which is a solution growth method using high-pressure ammonia as a solvent, has been used to achieve successful growth of real bulk GaN ingots [5]. Ammonothermal growth has the potential for growing large GaN crystal ingots because high-pressure ammonia has advantages as a fluid medium including high solubility of source materials, such as GaN polycrystals or metallic Ga, and high transport speed of dissolved precursors.
Currently, state-of-the-art ammonothermal method [6-8] relies on seed crystals to produce large ingots. A lack of large seed crystals free of strains and defects limits the growth of high quality bulk GaN ingots with a diameter of 3″ or greater. Several potential seeds produced by different methods exist; however the seeds tend to be either small or defective. For instance, 2″ free standing GaN wafers have been produced by the Hydride Vapor Phase Epitaxy (HVPE) on sapphire or SiC substrates. Due to the large lattice mismatch between GaN and the sapphire or SiC substrates, the resulting GaN growth is bowed, strained and has a large defect density. Continued growth on a free standing seed produced by HVPE typically produces defective growth. In contrast, GaN crystals produced by the high pressure synthesis or sodium flux method tend to have high quality but limited size and availability. A method to improve defective seed crystals would improve the feasibility of producing large ingots suitable for use as substrates for devices.